Wide bandgap semiconductors (WBG) offer the a compelling solution for power electronics owing to their large breakdown electric field (Eb) and high carrier mobilities. The Baliga's figure of merit (εμEb3) of WBGs including gallium nitride (GaN) and silicon carbide (SiC) is approximately one hundred (100) times higher than that of silicon (Si), and the figure of merit for WBGs with wider bandgap such as aluminum nitride (AlN) and diamond is even higher. WBG power electronic devices promise miniaturized power supplies and agile electricity conditioning systems with higher efficiency than the present technologies based on Si. Furthermore, they promise operations under high temperatures and harsh environments such as near a car engine and a myriad of applications which cannot yet be foreseen today. Even though this fact has been known for several decades, the development of WBG power electronics has been long limited by two intrinsic difficulties: 1) Lack of large size crystalline substrates with low defect densities and 2) efficient doping of both n- and p-types.
The recent development of unipolar devices on WBG with relatively low crystal quality has been notable, with GaN-onSi high-electron-mobility transistor (HEMT) based lateral power switches being a prime example. However, for higher voltage and power applications, other devices, such as for example, vertical power devices are necessary, demanding high crystal quality and both n- and p-type doping. SiC as an indirect semiconductor enjoys conduction modulation to reduce on resistance (Ron) and due to the same very reason, the SiC power switches are slow. Also, substrates of large size (e.g., greater than approximately four inches) and low defect densities (e.g., less than approximately 102 cm−2) are available for SiC. On the contrary, heterostructures are more readily available in GaN. Combined with higher carrier mobility in GaN, higher output currents and, more importantly, higher efficiencies and higher frequency operation are expected.
The advent of high quality GaN substrates (e.g., threaded dislocation densities (TDDs) of approximately 104 cm−2) generally enables vertical GaN high power devices. But it does not solve the two intrinsic problems of GaN vertical switches compared to Si: a) inferior inversion channels, which is also an ongoing challenge for SiC, and b) poor p-type conductivity for both GaN and SiC, but no p-GaN is demonstrated yet by ion implantation.